The present invention relates to polarization velocity vector measuring apparatuses that measure polarization change characteristics with respect to, for example, optical fibers without being dependent on the states of polarization of measurement probe light, and to polarization velocity vector measuring methods that utilize such apparatuses.
In digital coherent reception technology, a high-speed polarization demultiplexing and waveform equalizing function are achieved so that reception of a modulation format using a polarization space has become possible. However, the state of polarization (SOP) of light input to an optical-fiber transmission path changes in response to a perturbation, such as slight vibration of this optical-fiber transmission path. This has to be dealt with by evaluating a change in the SOP of a received optical signal observed at the output end of the optical-fiber transmission path in a digital coherent transmission system.
The main factor for a change in the SOP observed at the output end of the long-distance optical-fiber transmission path is a temporal change in birefringence caused by stress distributed to the optical-fiber transmission path. Since the stress distributed to the optical-fiber transmission path changes irregularly due to, for example, a temperature change or vibration, the SOP observed at the receiving side changes randomly. Moreover, the rate of change in the SOP is not fixed, but changes statistically.
Due to the above reasons, in the evaluation of a practically-used optical-fiber transmission path, the possibility of observation of the rate of change in the SOP (i.e., rate of polarization change) and the possibility of emulation in a test room that provides an assumed environment similar to that of an actually-installed optical-fiber transmission path are important research issues for the evaluation technology of optical-fiber transmission paths.
In the related art, two methods have been employed as methods for quantifying a change in the SOP. The first method involves measuring a rate of temporal change ∂α/∂t (rad/s) in angle α=cos−1[sout(t)·sout(t+Δt)] formed by two SOP vectors sout(t) and sout(t+Δt) (for example, see L. Yao, H. Huang, J. Chen, E. Tan, and A. Willner “A novel scheme for achieving quasi-uniform rate polarization scrambling at 752 krad/s,” Optics Express, Vol. 20, No. 2 (2012), pp. 1691-1699 (which will be referred to as “Non Patent Literature 1” hereinafter)). In this case, the SOP vectors are Stokes vectors. The second method involves measuring ∂r/∂t=[∂σ/∂t)2+(∂θ/∂t)2]1/2(rad/s) based on an amount of change σ on the equator and an amount of change θ on the diameter in a coordinate system (1, σ, θ) of a Poincaré sphere having a radius of 1 (for example, see P. J. Leo, G. R. Gray, G. J. Simer, and K. B. Rochford, “State of Polarization Changes: Classification and Measurement,” IEEE Journal of Lightwave Technology, Vol. 21, No. 10, 2003, pp. 2189-2193 (which will be referred to as “Non Patent Literature 2” hereinafter)).